1. Field of the Invention
This invention relates to a method and apparatus for making strips, bars and wire rods of small cross-sectional areas, and more particularly to a method and apparatus for continuously casting sections of steel and other metals using an annular mold having an endless open-top casting groove and then rolling the cast sections into strips, bars and wire rods of small cross-sectional areas.
2. Description of the Prior Art
Sections having small cross-sectional areas can be continuously cast by use of a horizontal rotary groove mold.
This horizontal continuous casting method is suited for casting sections having small cross-sectional areas whose thickness is in the range of approximately 10 mm to 100 mm, not requiring heavy equipment investment while assuring high productivity. Typical examples of this method are disclosed in U.S. Pat. Nos. 3,284,859 and 3,478,810 and Japanese Patent Publication No. 13785 of 1988. The continuous caster disclosed in U.S. Pat. No. 3,284,859 has an annular mold having a trough or casting groove. The annular mold turns around a vertical shaft, and molten metal is poured from the tundish into the casting groove. To cool the molten metal in the mold, a forced cooling unit comprising spray nozzles disposed substantially at right angles to the mold wall is provided. The solidified section is continuously withdrawn from the casting groove at a point 200 to 270 degrees apart from the pouring point and delivered to the subsequent continuous rolling mill. Because of the open-top groove-shaped mold, the section cast by this method is forcibly cooled on three sides but not on the top. Thus cooled less than the other three sides, the top of the section being cast solidifies more slowly. The section cast by this method solidifies in this characteristic way. Therefore, the cast section must not be taken out of the mold until a solidified shell has been formed on the top thereof.
To take the cast section out of the horizontal rotary annular mold, the cast section must be straightened at least once. The cast section to be taken out of the annular mold must be lifted by some means. If left in the lifted position, however, the cast section will move diagonally upward beyond the straightener. Therefore, the cast section should preferably be vertically straightened again to make the pass line thereof horizontal. This is because the as-cast section does not have adequate mechanical properties and, therefore, needs a further rolling. Then, a horizontal pass line facilitates such subsequent rolling and delivery of the cast section to the heating furnace and other facilities therefor.
Lifted out of the mold, however, the section cast by this type of apparatus needs a combined application of horizontal and vertical straightening that can result in three-dimensional complicated torsional deformation. Because bending and torsion are the main stresses acting on the cast section, maximum stress works on the surface of the cast section, and, as a result of which, cracks tend to occur at the surface. Though varying somewhat with chemical composition and other factors, the embrittling temperature of carbon steels being cast is said to be normally in the range of 700.degree. to 1200.degree. C. This high-temperature embrittlement is said to be caused by the embrittlement of grain boundaries due to the phase transformation of steel and the precipitation of carbides, nitrides, sulfides, etc. It is therefore desirable to keep the surface temperature of the cast section out of the 700.degree. to 1200.degree. C. range during straightening. Actually, however, straightening in the continuous casting process with an annular mold having an endless open-top casting groove is normally performed in the temperature range of 700.degree. to 1200.degree. C. In an experiment conducted by the inventors, the temperature at the sides, bottom and their corners of the section being cast readily was dropped to approximately 700.degree. C. before straightening was applied while waiting until the top surface of the cast solidified in the mold. It was difficult to keep their temperature above 1200.degree. C. This embrittlement can be easily and effectively avoided by cooling the cast section to below 700.degree. C. But this method is undesirable because reheating for the subsequent rolling pushes up production costs. As such, it should be considered as a last resort to be employed when no other solution can be found.
To prevent cracking in the above embrittlement temperature range, it is essential to minimize straightening strain (or straightening stress). With the straightening of the section cast through an annular mold having an endless open-top casting groove, however, no definite conditions for the prevention of cracking have been disclosed. Therefore, it seems that maximum benefits can be derived from the continuous casting method being discussed when such conditions are established. They do not seem to have been very important so long as the method has been used mainly in the continuous casting of aluminum, copper and other nonferrous metals having very high deformabilities. But commercially applicable straightening conditions must be established for carbon steel and other similar materials whose ductility not only is relatively low but also changes radically with the casting temperature.
Furthermore, conventional continuous casting with an annular mold having an endless open-top casting groove has been of the single strand type. Meanwhile, a combination of continuous casting and subsequent direct rolling utilizing the sensible heat of the cast section is known to enhance productivity while lowering production cost. Enhancement of productivity and lowering of production cost can be achieved by increasing either the casting speed or the cross-sectional area of the cast section. In increasing the casting speed, however, the machine length, which, in turn, is limited by the completion time of solidification, must be considered. Therefore, faster casting calls for a larger caster. Casting sections of larger cross-sectional areas also necessitate a larger caster. But larger casters, which are more expensive than smaller ones, neither provide the benefit of low equipment cost, which is one of the main advantages of the method being discussed, nor permit savings production in costs. As such, an effective way to cast sections of smaller cross-sectional areas with a smaller caster is a multi-strand casting method in which a number of small sections are cast at a time.
In continuous casting apparatus, an annular mold having an endless open-top casting groove is rotated within a horizontal plane. Therefore, a dam to prevent the backward flow of molten metal (hereinafter called the tail dam) is provided upstream of the pouring point and a dummy bar or a member to prevent the outflow of molten metal (hereinafter called the front dam) is provided downstream thereof. Normally, therefore, casting is started by pouring molten metal into an initial pouring space formed by the tail dam and the front end of the dummy bar or the front dam, with the rotation of the mold being started when the poured molten metal in the space reaches the desired level. The height of the section to be cast is determined by the level of the molten metal and can be adjusted by varying the balance between the pouring and withdrawing rates. Of course, casting can be carried out without thoroughly filling said initial pouring space with molten metal. But such practice is not recommended as it would cause significant size variations in cast sections which, in turn, might lower the production yield and induce various rolling troubles.
When the casting method being discussed is carried out in a multi-strand fashion, more serious problems will come up. Because the concentrically disposed the casting grooves are rotated at the same speed (angular speed), casting speed must be differentiated with regard to the inner and outer strands. Therefore, production rate varies with strands when the sections are cast to the same cross-sectional area. When multi-strand casting is combined with direct rolling, an additional coordination between the two processes becomes necessary. Moving together with the mold, the dummy bar or front dam determines the shape of the leading end of the cast section. Connected to a stationary member isolated from the rotary mold, on the other hand, the tail dam remains in its original position until casting is complete. Therefore, the height of the section to be cast is determined by the level of the molten metal and can be adjusted by varying the balance between the pouring and withdrawing rates, as mentioned before. In multi-strand casting, the molten metal in the individual strands must reach the same or desired level at the same time because the individual molds are rotated by same drive mechanism. But it is practically impossible to make the pouring rates of all strands completely equal because the size of the initial pouring space in each strand is not necessarily the same and molten metal does not always flow in the same manner. Therefore some measure must be taken at the start of the casting. When completing casting, the rotation of the mold must be stopped to permit the shaping of the tail end (hereinafter called the top portion) of the cast section. After being thus suspended, the rotation of the mold is resumed when the top portion of the cast section has solidified (this solidifying process is called top processing). As the cast section is not taken out during the top processing, the temperature of the section being cast in the mold drops so much that casting and rolling utilizing the sensible heat of the section and the resulting energy saving are difficult to achieve. When carbon steel or an other similar type of steel is cast, the temperature of the cast section held in the mold for top processing falls into the aforementioned high-temperature embrittlement range, whereby cracks tend to occur in the cast section in the subsequent straightening process. As such, top processing must be completed without causing the undesirable stagnation of the cast section in the mold. Furthermore, the advent of appropriate outflow preventing member and dummy bar, suited for use in horizontal multi-strand continuous casting with an annular mold having endless open-top casting grooves and in other types of casting operations, has long been awaited.